Helium microprobe analysis of nickel silicide diodes

Nuclear Instruments and Methods North-Ilolland, Amsterdam

in Physics

HELIUM

ANALYSIS

MICROPROBE

J. THORNTON

Research

R29 (1987) 515-520

515

OF NICKEL SILICIDE DIODES

‘), R.E. HARPER ‘I*, and

D.M.

ALBURY

2,

I) Ueportmenr I$ Electronic and 1:‘leciricai Engineering ‘) Department

University of Surrey, Guildford, Surrey G(i2 5XH, England vj Physics, Universily of Surrey, Guild/or4 Surrey GU2 5X11, England

In this paper we present RBS and channelling measurements made on microscopic nickel silicide diode structures. These were obtained by using the helium ion microprobe at the University of Surrey. We also show that the new prc-lens deflection system enables measurements to be made with a 10 pm diameter probe over a 2 x 2 mm area without significant dcchannelling or increase in the probe’s diameter.

1. Introduction Rutherford backscattering (RBS) analysis provides elemental and depth information. The degrees of impurity substitutionality and matrix disorder can be measured using channeling [l]. These are both forms of ion beam analysis (IBA). Because 1 mm diameter beams are normally used these techniques are restricted to large uniform samples. By employing an ion lens it is possible to focus an ion beam to produce probes to carry out IBA on microscopic areas (less than 0.01 mm in diameter) in a practical time [2]. However, with many ion lens systems presently in use, the convergence of the beam is too large for MeV ion channeling. The full width half maximum acceptance angle for the (100) direction in silicon is 0.2 o for 1.5 MeV helium [ 11. The MeV ion microprobe beam line at the University of Surrey has recently been redesigned and rebuilt to produce a practical system. The new system has been described in detail elsewhere [3]. The ion lens employed is the Harwell quadruplet. It has three advantages: (1) The full beam convergence of the probe is only 0.16”. (2) Low aberration pre-lens deflection is possible in both directions perpendicular to the undeflected beam. (3) The deflected beam’s angle of incidence to the undeflected beam’s direction is small (O.l” for a 1 mm deflection). Microprobe channelling is therefore possible with the Surrey instrument. We report on experiments to test the microprobe performance, in particular the analysis of nickel and nickel disihcide diode test structures by * Present address: sion, Harwell gland.

Instrumentation and Applied Physics DiviLaboratory, UKAFA, Oxon OX11 ORA, En-

0168-583X/87/$03.50 0 Elsevicr Science Publishers (North-Holland Physics Publishing Division)

B.V.

micro-RBS and channelling. Nickel disilicide was chosen because it has been demonstrated [5] that this compound grows epitaxially on single crystal silicon substrates during electron beam annealing. The diode structures therefore provide a opportunity both to test the channelling capabilities of the microprobe and to investigate the possibility of epitaxial growth on a real device.

2. Apparatus The probe’s diameter was chosen by changing the object aperture, a selection of which were held on a hinged micrometer arm. The vertical movement, which changed the aperture, also served to position it vertically. The aperture’s horizontal movement was controlled by a second micrometer and its opposing spring which swung the aperture holding arm on its hinges. An optical microscope with a range of magnifications from x 1 to x 150 has been added to the target chamber. There were two reasons for this: firstly to aid in the alignment of the instrument and secondly to show the relative positions of beam and sample. The sample stage could hold several samples and the glass cover slips used for monitoring the beam. The sample to be analysed was selected by translating the sample stage. The probe was moved across the sample by the electrostatic deflection of the beam prior to its passage through the quadruplet. To achieve this without introducing significant aberrations, double deflection systems were used in both the vertical and horizontal directions [3,4]. The layout and action of these deflection systems are illustrated in fig. 1. The symmetry of the Hat-well quadruplet allows this system to be used in both directions [3].

516

Fig. 1. A double deflection scanning system which minimises the beam aberrations, caused by pre-lens deflection, by passing the beam through the intersection of the quadruplets axis and entrance principle plane [4].

3. Experimental The experimental work was divided into three (1) The measurement of the probe’s diameter. (2) The measurement of xmin for deflected and fleeted beams. (3) The RBS and ion channelling analysis of the structures. xrmn is the ratio of the channelled and random scattering yields [l].

parts: undediode hack-

Ion channelling spectra of different areas of the same silicon sample were then collected with the beam undeflected and deflected in the horizontal and vertical directions. Another series of consecutive spectra were taken to measure the rise of dechanneling with fluence. xmin was defined as the ratio of the channelled yield of the sample to that from amorphised material. The amorphised silicon sample had been produced by implanting a dose of 10” 400 keV Ge ions cm-‘.

3.3. Anat’ysis of the diode sampies 3. I. The probe diumefer The prohe’s position at the sample plane could he monitored by viewing the fluorescence produced when the beam struck a glass cover slip at the sample plane. Measurement of the probe’s diameter by this method was not possible because the size of the fluorescence and the diameter of the probe were different. We have instead scanned copper electron microscope grids while monito~ng the copper hackscattered signal. Because the grid’s pitch and bar thickness were known the probe diameter could be determined.

The structure of a 0.1 mm diameter diode is shown in fig. 2. Diodes of 0.07 and 0.03 mm diameters were also available on the same samples. The structures were manufactured by first etching windows in the oxide and then depositing a layer of nickel on and around the hare silicon. Some diodes were then electron beam irradiated for 60 s at either 4 W cmm2 or 8 W cmm2. These power densities produced Nisi and Nisi, respectively on macroscopic samples IS]. It is difficult to produce a channelled spectrum of a small feature on a semiconductor structure because unwanted surface damage can be produced by the beam

3.2. The measurement oj x,,, A channel was found in virgin (100) silicon with a 45 pm diameter probe by rotating the sample stage while rno~to~g the backscattered flux. The larger beam current (8 nA) available with the large diameter probe made the location of the channelliig direction easier because of the greater backscattered particle count rate. The sample stage was then translated to place the coverslip under the beam. This enabled the probe to he brought back to its previous aligned position when the object aperture was changed to produce a 10 pm diameter probe.

-loOpm-

Diode Cross-Section Fig.2. The cross-section of one of the larger diodes.

511

J. Thornton et al. / Helium microprobe analysis of nickel silicide diodes

during the processes of alignment and feature location. Therefore it is best to align the sample prior to bombardment. In order to allow this to be done for the diode sample, part of the sample was dip etched in HF to remove the oxide coating. The remainder of the sample was protected from the HF fumes by coating it in an acetone soluble varnish. This was removed after etching and before ion beam analysis. A large section of bare silicon was thereby made available for the initial location of the channelling direction. Once this was determined the probe was deflected onto the diode of interest. Without some sort of feedback with which to monitor the probe’s location, positioning the probe would have been hopeless. The X-ray and backscattered yields were too low to be used for this purpose. Fortunately the silicon oxide covering the samples around the diodes fluoresced when struck by the probe (as do quartz monitors). Therefore the probe’s movement across the sample could be controlled. Channelling measurements were also made on a macroscopic silicide sample with the 10 micrometer diameter probe. Random spectra of the diode structures, before and after annealing and the macroscopic nickel disihcide sample were also taken with the sample plate and beam in a non-channelling alignment.

4. Results The diameters of the probe obtained by deconvoluting the RBS profiles of the copper grids are shown in

Table 1 Probe diameter on deflection

Horizontal Vertical

Deflection (r@

Probe diameter (pm)

Conversion factor (mm/kv)

0

8*2 9+2 12*3

1.2 0.96

1.2 2

Table 2 Channelling on deflection

Horizontal Vertical

Deflection (nun)

Xmi” @)

0 1.9 1.5

4 4 4

table 1. The table also shows the “deflection potential difference to distance across the sample” conversion factors. Deflections of 1 mm, in either direction, did not produce a significant increase in the probe’s diameter. Similarly the values of xmin listed in table 2 show that deflections of 1 mm do not produce any significant dechannelling. Changing the polarity of the deflection potential has no effect therefore channelling analysis was possible with a pm diameter probe over a 2 x 2 mm area. The value of (4 f I)% for xmin is typical of single crystal silicon.

300

Surface

225

100

160

220

230 CHANNEL

340

NUMBER

Fig. 3. A random (A) RBS spectrum from one of the unannealed diode structures.

400

518

J. ~ornt~n

et al. / Hehn

microprobe analysis

ofnicket

Surface Si

silicide diodes

Surface Ni

1 I

r

I 4%

200

260 WANNEL

320 NUMBER

360

440

Fig. 4. The random(A)and aligned (B) RBS spectra from one of the annealed diode (8 W cm-z The channeling and random spectra for the diode and macroscopic samples are shown in figs. 3-5. Kotc the energy calibration is slightly different in each case. The increase of xmin with dose for the 10 ym probe and (100) silicon is shown in fig. 6. Dechannelling became significant at a dose off 0.2 C cm 2. This is in agreement with data from the Melbourne microprobe [6].

for 60 s) strucuires.

5. Discussion Comparison of the random spectra of fig. 3 (before annealing) and fig. 4 (after annealing) shows that the nickel and the silicon became mixed on annealing. Some of the nickel went deep enough into the silicon for the signals from the two elements to become superim-

Surface Ni

Surface Si

V-7 280 -_ CHANNEL NUMBER Fig.5.The random(A>and aligned (B) spectra from the annealed (8 W cm-

*

340

408

for 60 s) macroscopic silicide sample.

J. Thornton et al. / Helium microprobe analysis of nickel silicide diodes 100

1

.E

x’

0

1

2

3 DOSE

4

5

(X10-' I:m-21

Fig. 6. The rise in xmin of (100) silicon during bombardment with the 1.5 MeV 10 pm diameter helium microprobe. The probe was also the means of measuring xmi,.

posed. This is the origin of the peak at channel 250 in fig. 4. The more typical disilicide spectrum shown in fig. 5 was produced by analysing the macroscopic sample with the 10 pm diameter probe. The nickel and silicon signals do not overlap. Furthermore the nickel signal tails off abruptly indicating a sharp nickel sihcide to silicon transition. The 2: 1 ratio of the near surface nickel and silicon signals shows that nickel disilicide had been formed. If the nickel tail contribution is subtracted from the nickel plus silicon signal in fig. 4, the diode does appear to have the same stoichiometry near the surface. However, there is no sharp interface. The channelled spectrum of fig. 4 shows that there is no lattice match between the silicide and the underlying silicon (x,,,;, = 100%). That the crystal and beam were aligned is shown by the reduced yield in the silicon substrate signal despite the large amount of dechannelling produced by the beam’s passage through the silicide. The inability to channel in the diode sihcidc was shown to be due to inherent misalignment or disorder and not beam damage, by charmelling with the same probe on the macroscopic silicidc sample. x,,,,” of 10% were obtained for both the nickel and silicon signals from the macroscopic sample despite a larger helium dose and hence more beam damage having been produced. The ion channelhng spectra of the nickel disilicide diode structures were consistent with a polycrystalline nonepitaxial silicide being formed. The only difference other than size between the diodes and the macroscopic sample is the sample preparation. The macroscopic sample are made using a virgin silicon wafer which is

519

dipped in hydrofluoric acid to remove the native oxide prior to nickel deposition, whereas the diode structures arc fabricated by thermally oxidising the wafer then etching out windows over which the nickel is deposited. If the SiO, is not completely removed then it will inhibit the Nisi, growth producing a film with poor stoichiometry and crystalhnity, and with a very ragged interface. Such a film would yield RBS spectra as in fig. 4. Similar results have been obtained on macroscopic Nisi, and CoSi, samples with high levels of oxygen contamination [5]. A smaller diameter beam is required to investigate smaller features. To obtain similar statistics with a smaller probe, before beam damage becomes important, the detector solid angle must be increased. A commercially available annular detector, with an active area of 200 mm2, on the Surrey microprobe would allow 4 pm diameter probes to be used. Ion channelling is possible with a 1.5 MeV microprobe because the bulk of the lattice damage occurs at the end of the light helium ions ranges. The measurement of the substitutionality of dopants does not appear to be possible with a microprobe because sufficient statistics cannot be collected before the substitutional atoms are displaced. For example, during work with a 1 mm diameter 1.5 MeV helium beam, the number of arsenic atoms on lattice sites was observed to fall for flucnces greater than 1 mC cmm2 [7].

6. Conclusions In comparison to conventional channelling, microchannclling is a destructive technique. However, we have demonstrated that the Surrey microprobe can be used to produce channelled spectra of semiconductor structures from 10 pm features or larger over a 2 X 2 mm area. It has been employed to analyse microscopic nickel sihcide diode structures and important differences in the; stoichiometry, crystallinity and silicon-silicide interface roughness, between the diodes and macroscopic silicides have been found. The authors are grateful for the useful discussions with P.L.F. Hemment, N.M. Spyrou and J.M. Shannon who also provided the diode structures. We are also grateful to J. Mynard and J. Brown for providing good, stable helium beams. References [I] RR. Appleton and G. Foti, in Ion Beam Handbook

for Materials Analysis, eds., J.W. Mayer and E. Rimini (Academic Press, New York, 1977). [2] J.A. Cookson, A.T.G. Ferguson and F.D. Pilling, J. Radioanal. Chem. 12 (1972) 39.

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et ul. / fielium

microprobe

C. Jeynes. J. Thornton, A. Way, R.P. Webb, D. Albury, P.L.F. Hemment and K.G. Stephens, Nucl. Instr. and 1Meth. B6 (1985) 264. [4] D. Heck, Nucl. Instr. and Meth. 197 (1982) 91. [S] R.E. Harper, C.J. Sofield, I.H. Wilson and K.G. Stephens, MRS Proceedings 37 (1985) 573.

analysis o/nickel

silicide diodes

[6] S.A. Ingarfield, C.D. McKenzie, K.T. Short and J.S. Williams, Nucl. Instr. and Meth., 191 (1981) 521. [7] E.A. Maydell-Ondrusz, I.H. Wilson and K.G. Stephens, MKS Proceedings 45 (1985) 123.